1. Field of the Invention
This invention relates to thermoplastic elastomer blends of polymers that have been prepared by dynamic vulcanization. The polymer blends comprise a crystalline polyolefin plastic matrix, such as polypropylene, and two crosslinked elastomeric polymers wherein one elastomer is butyl or halobutyl rubber and the other is an ethylene-propylene copolymer-based rubber.
2. Background
This invention relates to polymer blends which have a combination of both elastic and thermoplastic properties and which are of significant commercial interest. Such thermoelastic compositions are generally obtained by blending an elastomeric composition with a thermoplastic composition in a way such that the elastomer is intimately and uniformly dispersed as a discrete phase within a continuous phase of the thermoplastic composition. These polymer blends have been given the generic designation of Thermoplastic Olefins ("TPO"). They exhibit some of the properties of a cured elastomer as well as the reprocessibility of a thermoplastic resin. The elastomeric characteristics are enhanced if one component of the blend is a vulcanizable elastomer which is wholly or partially cross-linked.
The earliest work in the curing of a TPO composition was by Gessler and Haslett; see U.S. Pat. No. 3,037,954. That patent teaches the concept of "dynamic curing" wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer is cured while continuously mixing and shearing the polymer blend. The resulting blend is a micro-gel dispersion of cured elastomer in an uncured matrix of resinous thermoplastic polymer which is known as a dynamically vulcanized alloy ("DVA").
Gessler, '954 discloses compositions comprising polypropylene and a rubber such as, inter alia, butyl rubber; chlorinated butyl rubber, polybutadiene, polychloroprene and polyisobutene. Compositions of about 50 to 95 parts polypropylene and about 5 to 50 parts of rubber are disclosed.
The commercially useful DVAs are known to include butyl-based DVA's and EP/EPDM-based DVAs. The tensile strength of butyl-based DVAs has typically been lower than that of EP/EPDM-based DVAs when compared at the same Shore A hardness or polyolefin resin content. Thus, efforts have been directed towards improving the physical properties of butyl-based DVAs.
U.S. Pat. Nos. 3,758,643 and 3,806,558 disclose TPO type polymer blends comprising an olefin thermoplastic resin and an olefin copolymer rubber wherein the rubber is dynamically cured to a partial cure state. The compositions are reprocessible and result in molded products having good surface appearance. However, the potential applications of such blends are limited by their high compression set and/or low softening temperature resulting from only a partial cure of the rubber. Furthermore, the partial peroxide cure utilized in such blends is difficult to control from the standpoint of completeness of reaction, resulting in batch to batch variations in product properties.
U.S. Pat. No. 4,639,487 to Hazelton, et al. is directed to heat shrinkable DVAs including an ethylene copolymer resin blended with a butyl or halogenated butyl rubber. The butyl rubber should be at least partially dynamically vulcanized to a cured state in the presence of the copolymer. The invention is restricted in that peroxide cure systems are specifically excluded. These DVA compositions are said to possess exceptional resiliency, high coefficient of friction surfaces and low compression set.
U.S. Pat. No. 4,212,787 to Matsuda, et al., however, allows the use of peroxide cure systems and is directed to the production of partially cured DVA compositions which include 40-100 wt. % of a peroxide curable copolymer (such as EPDM); 0-60 wt. % of a peroxide decomposing copolymer (such as PE and PP); and 5-100 wt. % of a peroxide non-curable rubber, either polyisobutylene or butyl rubber. One of the objects of the Matsuda invention is to produce a DVA having improved surface appearance. This is effected by improving the fluidity of the DVA, relative to blends as disclosed in U.S. Pat. No. 3,806,558, without degradation of the heat resistance, tensile strength, flexibility, rebound-elasticity, etc.
U.S. Pat. No. 4,202,801 to Petersen relates to the partial dynamic curing of a blend of a monoolefin copolymer rubber, such as saturated EPM or EPDM; a polyolefin resin such as PP or PE, with a conjugated diene rubber such as polybutadiene or polychloroprene. Crystalline polyolefin resin may be used. The cure systems useful for the invention include the peroxides. More than one monoolefin copolymer rubber, conjugated diene rubber and polyolefin resin may be used in combination. The DVAs of this invention are said to provide low compression set and high tensile strength at elevated temperatures.
U.S. Pat. No. 4,340,684 to Bohm, et al. discloses a DVA composition which is said to have very good physical properties, especially tear strength, tensile strength, elongation at break, low temperature impact resistance, minimum creep at high temperatures, and smooth surfaces when injection molded. The compositions, which may be partially cured or uncured, comprise a blend of from about 10 to about 50 wt. % of a crystalline 1-olefin polymer, from about 80 to about 15 wt. % of a styrene-butadiene rubber, and from about 5 to about 55 wt. % of a highly saturated elastomer. The 1-olefin polymer may be polypropylene. The highly saturated elastomer is selected from the class consisting of hydrogenated polybutadiene, polyisobutylene and copolymers thereof such as butyl rubber, ethylene-propylene rubber (EPM), copolymers of ethylene-vinyl acetate, copolymers of ethylene-ethylacrylate, ethylene-propylene-diene monomer (EPDM), a hydrogenated "triblock copolymer of styrene-butadiene-styrene" and combinations thereof.
U.S. Pat. No. 4,607,074 to Hazelton, et al. is directed to a thermoplastic composition which comprises a polyolefin resin and two rubber components. The first rubber component is selected from the group consisting of polyisobutylene and ethylene-propylene copolymer and ethylene-propylene-diene copolymer. The second rubber component is selected from the group consisting of halogenated butyl rubber and polychloroprene. The invention requires the use of a cure system which vulcanizes one rubber but not the other. This results in a TPO having good physical strength characteristics coupled with excellent processability, low hardness and low compression set suitable for use in the manufacture of molded and extruded articles such as gasketing materials, boot seals, tubing, and the like.
U.S. Pat. No. 4,480,074 discloses DVA compositions said to exhibit improved surface characteristics and fabricability wherein the compositions are prepared by blending an unvulcanized, but vulcanizable, monoolefin rubber with a blend containing cured polyolefin rubber with crystalline polyolefin and subsequently vulcanizing such that the final blend comprises about 15-45 parts by weight of crystalline polyolefin and 85-55 parts by weight of vulcanized rubber. EPDM is taught as both the vulcanized polyolefin rubber and the unvulcanized but vulcanizable rubber in the disclosed blends. Dynamic vulcanization utilizing peroxide cure systems, phenolic resin systems, phenylene-bismaleimide and diamine curatives, etc., is disclosed.
Japanese patent application 85,530/87 discloses a TPO composition having excellent mechanical strength, thermal stability, moldability, gas impermeability and damping characteristics. The TPO of the '530 application includes a crystalline polypropylene as a matrix and two elastomers: a bromobutyl rubber and an olefin copolymer rubber such as EPM or EPDM rubber. The composition also includes conventional additives such as process oil. All of the components are combined and vulcanized in a single batch with a peroxide cure system but there is no indication of the inclusion of a peroxide co-agent such as m-phenylene bismaleimide (HVA-2) or the like. The '530 application's inventors found that while butyl and chlorobutyl rubbers are not cross-linkable with peroxide cures, bromobutyl rubbers are. Moreover, the '530 application's inventors explain that the enhanced physical properties claimed are due to the olefin copolymer rubber which provides flexibility to the TPO and also acts as a binder at the interface between the polypropylene and the bromobutyl rubber.